Pressurized-water nuclear reactors comprise a core consisting of prism-shaped assemblies arranged side by side in vertical position. The assemblies comprise a framework which is closed by connectors and in which are arranged the fuel rods held by spacer grids spaced from one another in the longitudinal direction of the assembly.
The spacer grids form a regular network, in which some locations are occupied by guide tubes intended for receiving the absorbent rods of control clusters ensuring control of the power released by the core of the nuclear reactor. At least some of the guide tubes are connected to the two connectors of the assembly by means of their end parts and ensure the junction between the framework components and the rigidity of the framework.
One of the connectors of the assemblies, called the lower connector, comes to rest on the lower core plate, which is pierced with holes in the region of each of the assemblies in order to allow the cooling water of the reactor to pass through the core in the vertical direction and from the bottom upwards.
The cooling fluid for the fuel rods passes through the adapter plate of the lower connector via orifices called water passages. Debris which may be present in the primary circuit of the reactor is liable to be carried along by the pressurized water in circulation, and when it is of small size (for example, smaller than 10 mm) this debris can pass through the adapter plate of the lower connector, the water passages of which have a large cross-section. This debris can become jammed between the fuel rods and the cells of the first grid, i.e., of the lowermost spacer grid holding the rods in the form of a regular network. This debris, subjected to the axial and transverse hydraulic stresses which are high in this zone, can wear the jacket of the fuel rod. This may result in a loss of sealing of this jacket and an increase in the rate of activity of the primary circuit of the reactor.
Devices making it possible to filter the cooling fluid while the reactor is in operation are known in the art. Such filter elements are associated with the fuel assemblies and are generally arranged in their lower connector. They generally consist of sheet-metal or metal-wire structures making it possible to detain debris having a size smaller than the largest dimension of the passage cross-section between a fuel rod and a grid cell.
Such filter elements are described, for example, in U.S. Pat. Nos. 4,664,880, 4,684,496 and EP-A-0,196,611.
Such devices can be complex and introduce a relatively high head loss into the circulation of the cooling fluid through the fuel assembly.
Moreover, these devices placed in the lower connector of the assembly can be bulky and obstructive during the loading and unloading of core assemblies and during the dismounting and refitting of the connections of the guide tubes and lower connector.
To deal with these problems, applicants' patent application FR-A-89-04840 therefore proposed a lower connector for a fuel assembly comprising a device for the retention of particles contained in the cooling fluid of the reactor, consisting of a filtration plate pierced with holes and fastened against the lower face of the adapter plate over a substantial part of its surface.
The adapter plate of such a connector comprises, in general terms, water passage orifices of cylindrical shape and of circular cross-section which extend through the adapter plate of square shape in the form of a square-mesh network. The filtration plate comprises sets of orifices of small dimensions which come into alignment with the water passage holes of the adapter plate when the filtration plate is put in place against the lower face of the adapter plate.
The arrangement of the water passage orifices in the form of a regular network and the arrangement of the holes of the filtration plate in coincidence with the water passage orifices, as described in patent application FR-A-89-04840, make it possible to obtain a water passage flow in the fuel assemblies which is maintained at an acceptable level while the reactor is in operation.
Nevertheless, it is still desirable to have the possibility of substantially increasing the flow of the primary fluid consisting of the pressurized water passing through the adapter plate of the lower connectors of the fuel assemblies.
It is likewise desirable to make it easier to machine the adapter plate and to limit the deformations of this plate as far as possible when the connector is assembled by welding.